Recently, the number of combustion plants using various heavy oils and coals as fuel has been increasing. These heavy oils and coals, containing a large amount of sulfur (hereinafter S), generate a large amount of sulfur dioxide (hereinafter SO2) and sulfur trioxide (hereinafter SO3) when they are simply combusted.
In particular, SO3 becomes sulfuric acid (hereinafter H2SO4) when water vapor (hereinafter H2O) exists in the same gas. For this reason, SO3 is specified as a designated substance by Air Pollution Control Law (in Japan) as one of causative substances of acid rain.
In combustion plants, a neutralizing agent such as ammonia (hereinafter NH3) is added to flue gas which is discharged from the plant to create solid neutralization products, which is removed with a dust arrester, so that the concentration of SO3 in the flue gas is lower than or equal to a specified value. However, since it has been difficult until now to continuously measure SO3, the injection amount of NH3 could not be optimally controlled. When the injection amount of NH3 is too large, it creates a large amount of ammonium sulfate, which clogs the dust arrester. In contrast, when the injection amount of NH3 is too small, it causes corrosion by H2SO4. Hence, the plant could be continuously operated only for a short period.
Measurement analyses of SO3 which have been conventionally conducted include: a method in which the gas is sampled using wet sampling, and all the amount is retrieved as H2SO4, which is then subjected to chemical analysis (including also a liquid chromatography analysis); and a method in which gas sampling is performed, and after removing dust, optical analysis is performed with a long-wavelength mid-infrared (around 7 μm) spectroscopy such as a Fourier transform infrared spectroscopy (FT-IR).
In addition, in recent years, a gas analysis device capable of continuously analyzing SO3 using a long-wavelength mid-infrared quantum cascade laser (hereinafter QCL) after gas sampling has been developed, studied, and sold in the market (for example, refer to Non-Patent Document 1 below).